This invention relates to magnetic bearings and methods for providing electrical energy to the magnetic bearings. More particularly, this invention relates to active magnetic bearings in which the control currents to the bearings are supplied by one or more generators, such as brushless generators. It may also be preferable to utilize brushless generators having air-core coils in place of iron-core windings in regions of the generators that are exposed to high frequency magnetic fields to provide even greater amplification of the supply energy to the magnetic bearings.
The use of magnetic bearings is well known. Magnetic bearings rely on attractive forces generated within a gapped magnetic circuit to generate forces on a supported rotating object (e.g., an iron shaft or disk). In traditional magnetic bearings, several energized ("biased") electromagnets are placed around the rotating object. All of the electromagnets are typically energized because the control coils of the magnetic bearings are inherently inductive, which directly affects the rate at which changes can occur in the electromagnetic forces. As the object moves out of a predetermined alignment position, the energy supplied to one or more selected electromagnets is increased, causing the rotating object to be pulled back into alignment.
The biasing of the electromagnets may be performed in several different ways. For instance, many magnetic bearings include biasing circuits which constantly apply bias current to the coils, thereby energizing the magnetic circuits to create a relatively small constant bias field in the bearing (for example, the constant field may be set at about 0.5 Tesla). It is also known to instead bias the electromagnets using permanent magnets to create the constant bias field, in which case the coils are only used by the control circuits.
One disadvantage of known active magnetic bearings is related to the delivery of control energy to the bearings. In some typical active magnetic bearings, as much as 100,000 watts of peak power must be supplied to the electromagnets in order to maintain the proper position of the rotating object (for example, large turboexpanders, large compressors, jet engines, etc.). To efficiently provide such a large amount of energy, the supply circuits are traditionally based on AC power, including massive electronic amplifiers. Additionally, the electromagnetic circuits require DC power, and thus, large, high-power, electronic switches are also required. The amplifiers and switches add substantial mass and volume to the active bearings systems, as well as substantial cost. For example, a typical DC power supply, electronic amplifier array and uninterruptible power supply (UPS) (in case of power failure) may cost upwards of one dollar per watt. Therefore, a 100,000 watt magnetic bearing would require a $100,000 power supply system.
One proposed solution to the cost, size and complexity of permanent magnet and electromagnet systems is the use of superconductive materials. For example, McMichael et al. U.S. Pat. No. 5,177,387 describes magnetic bearings in which high temperature superconducting material is utilized to provide support to a rotating member. However, these magnetic bearings still require the use of liquid nitrogen to cool the superconducting material such that it will perform adequately. As such, superconducting magnetic bearings may also be complex and costly systems to provide.
In view of the foregoing, it is an object of the present invention to provide improved active magnetic bearings in which energy is supplied to the bearing in an improved manner.
It is also an object of the present invention to provide improved active magnetic bearings in which the size of the power supply for the bearing is reduced.
It is a further object of the present invention to provide improved active magnetic bearings which may be produced at a reduced cost when compared to present magnetic bearings.